In computed tomography a two-dimensional image of a subject under examination is generated from multiple one-dimensional projections. A source of X-radiation transmits a beam of radiation through plural co-planar paths defining a cross-section of the subject under examination. The intensity of the attenuated radiation passing through the subject is monitored by one or more detectors whose electrical output is indicative of the radiation intensity impinging on the detector. The detectors are often single scintillation crystals, each optically coupled to a different photodiode.
So-called fourth generation CT scanners include a stationary array of detectors and a moving X-ray source. The detector array typically surrounds a patient aperture or scan circle which defines a patient scanning plane. The X-ray source when energized radiates the plane from a number of different directions as it orbits about the scan circle and the detectors measure the X-ray intensity of attenuated radiation passing through the patient.
The current fourth generation CT scanner designated as the Synerview 1200 is commercially available from Picker International, Inc. of Highland Heights, Ohio.
The radiation source typically employed in fourth generation CT scanners produces a diverging cone of radiation. A mechanical collimator is generally employed in conjunction with the source to limit the divergence of the cone to a fan-shaped swath which is confined to the patient scanning plane and to an angle which encompasses the subject under examination.
The X-ray beam is emitted from a small area on an X-ray tube anode known as the focal spot. A cathode assembly emits and directs electrons toward the anode. The trajectories of electrons are shaped into a beam of closely controlled dimensions. When a high voltage potential is applied to the X-ray tube, the electrons strike the anode in an approximately rectangular pattern. As a practical matter, however, X-radiation is produced at other locations other than the focal spot. Off-focal radiation is usually generated by secondary electrons that have been reemitted from the focal spot and return to the anode at points remote to the focal spot and produce X-rays at their point of impact. As many as 50% of the primary electrons are backscattered of which roughly one-half have an energy close to the primary beam and thus capable of producing off-focal radiation. In CT scanning systems, three to eight percent of the radiation detected has originated at points other than the focal spot.
The result is a broad low level X-ray source which projects its radiation to all points that are unshielded or uncollimated thence through any object to be radiographed. Small objects or sharp edges lose any proper definition because of the diffuse source. The lack of definition gives rise to non-linear artifacts in the reconstructed image. The mechanical collimator described above is able to occlude some but not substantially all of the off-focal radiation. The mechanical collimator is least effective in attenuating off-focal radiation in the central portion of the emergent beam.
In the above-described CT scanning systems, it has been found desirable to shape the intensity distribution of the X-ray beam across its angular extent. Shaped attenuation filters or compensators are currently employed and typically consist of wedges or contoured sections of plastic, aluminum, copper or composites of various material. These devices shape the radiation intensity of the beam to better match the attenuation characteristics of the subject under examination by absorbing radiation in a spatially controlled manner. By shaping the beam intensity in this fashion, the total dose to the subject under examination is reduced. The overall system radiation level can be increased to achieve a decrease in system noise since the range of radiation intensity emanating from the object under examination is reduced so as not to exceed the dynamic range of the detectors. These devices bear names such as dodgers, dose compensators and bow-tie filters.
A disadvantage of interposing such an element in the X-ray beam is X-ray spectrum modification by photoelectric absorption or beam hardening. Another disadvantage is the production of unwanted scatter radiation by the coherent and incoherent interactions of the filter with the X-ray beam. These secondary effects may influence the fidelity of the image reconstruction process, hence the quantitative accuracy of the CT scanner.
An additional phenomenon occurs due to the broad low-level radiation source (off-focal radiation). Small objects under examination are particularly susceptible to an unusual artifact which occurs due to interactions with an X-ray beam compensator and the off-focal radiation. For example, the image of a small cylindrical object, centrally located in the scan circle, changes from a circle with a circular central artifact to a crescent-shaped artifact, as the object is moved to the periphery of the scan circle. This phenomenon makes it very difficult to derive any quantitative information from the CT number of the image of the scan of such an object. The quantitative accuracy of the CT numbers that represent the image is degraded since the X-ray paths whose intensity is measured by the detector do not all emanate from a single point. Due to the off-focal radiation, the ray paths to the detector cut a broader path through the object than that accounted for by the reconstruction algorithms, and hence are less faithfully reconstructed.
Alterations in the reconstruction algorithm of compensate for these difficulties is complicated by the fact that the projection of the off-focal radiation varies depending on the position of a detector in the X-ray fan beam and whether a dose compensator is present. For example, off-focal radiation from the left most side of the anode is occluded by the left most edge of the primary collimator. However, the primary collimator does not occlude any off-focal radiation for a centrally located detector. Similarly the ray paths through a compensator are unequally attenuated through different portions of the X-ray fan. Any mathematical characterization of this phenomenon is complicated.
It is an object of this invention to provide a radiation attenuator which substantially eliminates off-focal radiation emanating from the radiation source and which compensates the beam profile without substantially changing the energy spectrum of the beam or creating substantial quantities of scatter radiation.